This invention relates to means for providing output signals indicative of information or data present on a medium and, more particularly, to circuitry for improving the reliability and accuracy of the output signals produced by such means.
With the advent of computers and various forms of data communication, coded information is placed on media such as tapes, badges or cards. This information is in the form of information characters located at sites of a grid or matrix selected in accordance with a code indicative of the particular information or data being recorded. The information characters are usually binary and have two states. Therefore, the medium may be altered at the locations of selected sites to indicate one state while the remaining unaltered sites represent the other state. There are many forms of information characters such as, for example, printed ink marks, magnetic domains, or punched holes.
Although these information characters may be read with the medium stationary by employing as many detectors as there are sites, dynamic reading with the medium moving past the detectors or vice versa offers the advantage of successive operation of a fewer number of detectors. For instance, when the sites form a matrix with columns and rows, only as many detectors as there are rows may read successive columns serially to obtain all the information on the medium. Conversely, a number of detectors corresponding to the number of information sites in a column may obtain all the information by successive reading of rows serially. In dynamic or successive operation of the detectors, mechanical arrangements often provide the relative motion between the medium and the detectors which is also known as scanning. The scanning motion may also be provided by manually moving the medium by an individual who is also operating the information reader.
In either case and particularly in the latter, a phenomenon known as skewing occurs. Skewing is the uneven and sometimes sporadic, at its worst, activation of the individual detectors as opposed to the simultaneous operation of the plurality of detectors in indicating the presence of information characters in a given column or row. Skewing is due to a number of factors some of which are irregularities in the information characters, different sensitivities of the detectors, variations in the tolerances of the mechanical arrangement to guide and constrain the motion, and variations in the dimensions of media, e.g., cards, badges or tapes. Uneven or irregular movement, such as that produced by manual sliding, severely aggravates the problem of skewing. Since all of the detectors must provide a correct and stable output signal at a predetermined point in time, usually indicated by a read or strobe pulse, to provide useful information for associated circuits and devices, skewing of output signals in accordance with the operation of associated detectors must not be tolerated.